The Radiation Field in Space

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Radiation is an acknowledged primary concern for manned spaceflight and is a potentially limiting factor for long term orbital and interplanetary missions. Results from numerous space probes demonstrate heightened radiation levels compared to the earth's surface and a change in the nature of the radiation field - particularly the presence of high energy heavy ions. Three sources of radiation are present.: solar cosmic rays, galactic cosmic rays and Radiation Belt particles.

Solar cosmic rays (SCR) originate from magnetically disturbed regions of the sun that sporadically emit bursts of charged particles with high energies. These events (solar flares) are composed primarily of protons with a minor component (5-10%) being helium nuclei (alpha particles) and an even smaller part (1%) heavy ions and electrons. Solar particle events develop rapidly and generally last for no more than some hours, however some proton events observed near Earth may continue over several days. The emitted particles can reach energies up to several GeV and arrive on Earth in tens of minutes to several hours depending on their energy. Anomalous large events with potentially life threatening consequences occur usually at the beginning or the end of maximum solar activity. During the last three 11-year cycles of solar activity in total four of those events were observed. Doses as high as 10 Gy could be received in a worst case scenario and would be lethal to humans within a short time. No method exists that can predict time of occurrence, frequency, intensity and duration of such events. Their buildup can be detected approximately ten minutes before it occurs, since there is a dramatic increase of visible light, x-rays and radiofrequency radiation. High solar activities may become visible in polar regions through the interaction of high numbers of solar particles with the atmosphere (Aurora). This is possible, since at the polar regions particles of any energy may enter parallel to the field lines.

Galactic cosmic radiation (GCR) originate outside the solar system. They have their origin in previous cataclysmic astronomical events such as supernova explosions. Detected particles consist of 98% baryons and 2% electrons. The baryonic component is composed of 85% protons (hydrogen nuclei), with the remainder being alpha particles (14%) and heavier nuclei (about 1%). All elements and energies are present. During high solar activity the solar wind is stronger and so are the magnetic fields transported by it, resulting in a decrease of the cosmic ray flux. The reverse is true when the solar activity is low. The modulation is effective for particles with energies below some GeV per nucleon. For increasing solar activity the maximum of the energy spectrum is shifted to higher energies. At 100 MeV per nucleon the particle fluxes differ by a factor of about 10 between maximum and minimum solar activity conditions, whereas at about 4 GeV only a variation of about 20% is observed. In low earth orbit there is also a strong latitude effect observed. The earth's magnetic field has the effect of shielding cosmic particles most effectively at the magnetic equator, thereby allowing only particles of high energy to enter the atmosphere, this shielding decreases towards the magnetic poles, allowing lower energy particles to reach the atmosphere as well. The radiation field around the Earth comprises a third radiation source, the Van Allen belts, which are a result of the interaction of GCR and SCR with the Earth's magnetic field and the atmosphere. The radiation belts consist of electrons and protons, and some heavier ions, trapped in the magnetic field. Of special importance for low earth orbits is the so called 'South Atlantic Anomaly' (SAA), a region over the coast of Brazil, where the radiation belt reaches down to altitudes of 200 km. This behavior is due to an 11° offset of the Earth's geomagnetic dipole axis from its axis of rotation and a 500 km displacement towards the Western pacific, with corresponding significant reduced field strength values. Almost all radiation received in low earth orbit is due to passages through the SAA.

The radiation field around the Earth comprises a third radiation source, the Van Allen belts, which are a result of the interaction of GCR and SCR with the Earth's magnetic field and the atmosphere. The radiation belts consist of electrons and protons, and some heavier ions, trapped in the magnetic field. Of special importance for low earth orbits is the so called 'South Atlantic Anomaly' (SAA), a region over the coast of Brazil, where the radiation belt reaches down to altitudes of 200 km. This behavior is due to an 11° offset of the Earth's geomagnetic dipole axis from its axis of rotation and a 500 km displacement towards the Western pacific, with corresponding significant reduced field strength values. Almost all radiation received in low earth orbit is due to passages through the SAA.

The radiation exposure of astronauts inside the ISS during solar minimum conditions arrives at 1 mSv/day ( the mean exposure on ground level is 2.5 µSv/year). During space walks this exposures are by at least a factor of three higher. Astronauts belongs therefore to the highest exposed persons. There are special radiation protection guidelines which limit the exposure per year to 500 mSv and to 1Sv (Europe) for the whole career.